In pyrometry, the temperature of a surface can be determined by measuring the radiation emanating from the surface. The emanating radiation will usually contain two components: the radiation emitted from the surface and radiation from other sources which is reflected off the surface.
Kirchoff's law states that emissivity of a surface plus reflectivity of a surface is equal to 1. For a theoretical black body, the emissivity is equal to 1 and the reflectivity is equal to 0. A black body is a theoretical object, that emits energy with complete efficiency at all Wavelength, and absorbs all energy directed at it at all wavelengths and therefore has no reflectivity.
A black body cannot exist in reality, because no surface radiates or absorbs all energy. A gray body is defined as an object which has a constant emissivity and therefore constant reflectivity at all wavelengths. As the emissivity of a gray body remains constant over various wavelengths, the emissivity of a gray body object is usually known or can be determined. From the emissivity, the temperature of the object can be determined by solving a Planck formula for temperature.
A nongray body is a body which has its emissivity vary depending on the wavelength measured. Therefore, to accurately determine the temperature of a nongray body using conventional pyrometry techniques, the emissivity of the nongray body must be known at the wavelength at which the pyrometer is operating.
A conventional single color pyrometer measures the radiation emitted from an object to be measured. As the radiation contains both an emission component from the object being measured and a reflection component containing radiation reflected off the object being measured, the emissivity of the measured object must be known to obtain an accurate temperature measurement. However, a single color pyrometer might be able to ignore the reflectivity of a surface to be measured if its reflectivity is small when compared to its emissivity.
Two color pyrometers measured the emitted radiation at two wavelengths and assumes that the ratio of emissivity at the two wavelengths is known. Therefore, the temperature is determined without knowing the emissivity of the object to be measured. In general, as long as the emissivity of the object to be measured does not change rapidly with wavelength, a fairly accurate temperature measurement might be obtained using the two color method.
However, both the one color and two color pyrometry methods are susceptible to errors caused by reflected radiation. Reflected radiation originates from sources other than the surface under consideration and reaches the pyrometer through reflection from the surface. Ceramics are a class of material which typically have low emissivity, and hence there is a high reflectivity in the band where conventional pyrometers operate. Consequently, a potential for large error exists when conventional pyrometry is employed to measure the temperature of ceramics.